Significance of continuous precipitation during creep of a powder mettallurgy aluminum alloy

Experiments were conducted to evaluate the creep properties of a 2024 aluminum alloy fabricated by powder metallurgy processing. The creep curves exhibit a minimum creep rate followed by an extended tertiary stage prior to failure. Using the values of the minimum creep rates, the apparent stress exponents are high and variable suggesting the presence of a threshold stress. Observations using transmission electron microscopy (TEM) reveal the occurrence of a continuous precipitation of fine particles during the tests. Although the density of these particles is dependent upon the testing conditions, quantitative measurements show that their average size is of the order of ∼ 60 nm under all conditions. A temperature compensated time is introduced to describe the evolution of fine particles during testing, and this permits the development of a relationship which can be used to estimate the density of the particles under any selected conditions.